The invention relates to a filler containing easily oxidizable elements such as aluminum (Al), titanium (Ti), zirconium (Zr), rare earths as well as a powder combination containing fluorides and carbonates for fusion welding of steel, nickel and cobalt alloys and corresponding castings containing easily oxidizable elements.
Welded joints are required to have at least the mechanical-technological quality of the materials to be joined. The composition of filler and base metal is decisive for the chemical composition--and consequently for the mechanical-technological quality--of a welded seam.
Heat resistant and extremely heat resistant alloys (base metals) are known which contain in addition to high percentages of nickel, chromium, molybdenum and cobalt also easily oxidizable alloy percentages of aluminum and/or titanium. In that case, the aluminum provides the oxidation resistance necessary at high operating temperatures, for example, the NiCr23Co12MO material (mat. no. 2.4663) alloyed with 0.8 . . . 1.5% Al.
Alloys which can harden by precipitation and the martensitic-hardening steels contain even higher titanium and/or aluminum percentages, for example, mat. no. 2.4654 with 1.2 . . . 1.6% Al and 2.8 . . . 3.3% Ti, i.e. mat. no. 1.6356 (X2 NiCoMoTi 18144) with 1.5 . . . 2.0% Ti.
These alloys can only be fully utilized in practice if they can be welded according to the same nature, in other words, the filler provides a weld in its composition having the same analysis as the base metal.
This type of extremely heat resistant alloy having easily oxidizable elements is known from DE-OS No. 22 38 609 which can be used as base metal and as filler. In this case fusion of the filler takes place according to the MIG method (metal electrode inert gas welding) under argon. Oxidation of the easily oxidizable elements is prevented as a result of this argon atmosphere and a weld having the same analysis is obtained.
It is, moreover, known from EP-PS No. 00 46 38 to fuse a base metal having easily oxidizable elements with a sheathed stick electrode with a core wire having the same analysis as the base metal. During welding, a burning off of the easily oxidizable elements takes place so that, for example, based on a core wire with 1.2% aluminum only residues of about 0.25% arrive in the weld as a result of oxidation. The produced low oxidation resistance of the weld, according to EP-PS No. 00 46 38, is compensated for by the addition of chromium, magnesium and niobium. The welding process with a sheathed stick electrode known from EP-PS No. 00 46 38, therefore, does not result in a weld having the same analysis as the base metal as a result of the oxidation of the easily oxidizable elements.
Tests have shown, moreover, that a weld having the same analysis cannot be attained even with the addition of the easily oxidizable elements in the sheath of the stick electrode. Percentages of about 0.5% of these elements are only attained in the weld whereby for such a low result, completely unsatisfactory welding properties (high ejection, uncontrolled melting of the crater) must be accepted.
Based on knowledge gained in MIG welding under argon in which oxidation is prevented by keeping oxygen away, also in welding with a sheathed electrode resp. with filler wires or in UP welding (submerged arc welding), the easily oxidizable alloy elements in the filler could be protected by using only oxygen-free components.
Among the materials currently used in the filler production, the following materials would accordingly be disadvantageous: the alkaline earth carbonates and their oxides as well as quartz (silicates), rutile (titanates), zirconium oxide (zirconates) and all heavy metal oxides.
Aluminum oxide and the aluminates are an exception based on their high formation heat.
The following oxygen-free components meet the stated requirements: alkali and alkaline earth fluoride including the complex fluorides, for example, cryolite.
An exclusive use of fluorides leads, however, to considerable problems with respect to welding technology. With sheathed stick electrodes, for example, the low melting point of the sheath is a disadvantage for a good crater formation at the end of the electrode. The resulting welding with a long light arc again leads to oxidation of the easily oxidizable elements from the arc atmosphere. Even the use of oxygen-free components, therefore, does not lead to a weld having the same analysis as the base metal.